The heart of a computer's long term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic bits to and reading magnetic bits from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal resulting in a low resistance state and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized resulting in a high resistance state. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause voltage changes that are detected and processed as playback signals.
In the ever increasing push for increased data rate and data capacity, engineers and scientists have continually found ways to make magnetoresistive sensors ever smaller. However such sensors are rapidly approaching a limit beyond which further reduction in size cannot be achieved. This is due in part to thermally induced fluctuations of the magnetization direction of the magnetic layers and in particular on the free layer magnetization in a Giant Magnetoresistance (GMR) or similar sensor. Thermal agitation becomes more severe as the sensor becomes smaller and the volume of the magnetic layers decreases accordingly. The magnetization fluctuation within the layers results in an increased sensor noise. Another form of noise that limits the extension of some sensors to small dimensions is present in GMR devices operated with the current perpendicular to the plane of the layers called spin torque noise that also contributes to the noise and reduces the signal to noise ratio of such devices. Other types of sensors that use magnetic layers have been investigated, including magnetic tunnel junction (MTJ) heads. Just like GMR heads, the MTJ heads exhibit magnoise and spin torque noise, both of which increase as device dimensions are made smaller. MTJ sensors also exhibit shot noise. With decreasing dimension eventually these noise sources will increase sufficiently to render many types of sensor unusable. Therefore, there is a need for a sensor that does not require the use of magnetic layers, and more specifically does not employ a magnetic free layer.
In order to develop such a non-magnetic magnetoresistive sensor, researchers have recently begun investigating what is referred to as the extraordinary magnetoresistive (EMR) effect. EMR theory is described by T. Zhou et al., “Extraordinary magnetoresistance in externally shunted van der Pauw plates”, Appl. Phys. Lett., Vol. 78, No. 5, 29 Jan. 2001, pp. 667-669. An EMR sensor for read-head applications is described by S. A. Solin et al., “Nonmagnetic semiconductors as read-head sensors for ultra-high-density magnetic recording”, Appl. Phys. Lett., Vol. 80, No. 21, 27 May 2002, pp. 4012-4014.
An EMR sensor operates based on the Hall Effect. When a charge carrier, such as an electron is moving through a material in the presence of both an electrical field and a magnetic field, the electron will be subject to a force along the electric field and a force given by the cross product of its velocity and the magnetic field. Thus the magnetic field tends to deflect the movement of carrier away from the direction of its motion. In some Hall devices that operate in a steady state, the carriers flow at an angle (called the Hall angle) with respect to the electric field given by tan(theta)=(Mu)×(B), where Mu is the material's mobility and B is the magnetic field. Some semiconductors can be made with Mu as large as about 60,000 cm2/Vs (=6/Tesla). At a magnetic field of 1 Tesla a Hall angle of 81 degrees can be achieved between the electric field and current flow resulting in a substantial change in the direction of motion of the carriers in a magnetic field.
An EMR device in its simplest form consists of an Ohmic bilayer structure of a metal and a high mobility semiconductor. When a pair of current leads are connected to a surface of the semiconductor at either end of the semiconductor, the current will tend to flow through the semiconductor to the more conductive metal (located opposite the current leads). The current will then travel readily through the more conductive material and then back through the semiconductor to the other current lead. When a magnetic field is applied perpendicular to the plane of the device, the Hall Effect will deflect the electrons so that some of them travel a longer distance through the more highly resistive semiconductor thus increasing the overall resistance of the device. This results in an increased resistance, which can be read as a voltage difference across the semiconductor, measured by voltage leads located on the same surface as the current leads. Thus the magnetoresistance of the device can be defined as the change in voltage between the voltage leads dVvv divided by the voltage applied to the current leads Vii, orMR=dVvv/Vii. 
Additionally, resistances for the voltage leads Rvv and current leads Rii can be defined by dividing through by whatever current is flowing through the structure, so thatMR=dVvv/Vii=dRvv/Rii. 
While such EMR devices provide the advantage of sensing a magnetic field without the use of a magnetic layer such as a free layer, EMR devices have not yet been used in disk drive devices. This is because other magnetoresistive sensors such as GMR sensor have provided sufficient sensitivity and bit resolution for bit sizes used so far. But as bit sizes narrow GMR and other sensors will be unable to achieve the necessary resolution, making an alternative necessary. Particularly, the bit size and track density required for ultrahigh density magnetic recording present formidable challenges for current-art magnetic sensors and servo schemes. For example for recording at 1 Tb/in2, the minimum domain size will be either a circular mark of ˜15 nm in diameter or a rectangular ˜15 nm×15 nm bit. The track spacing required to meet this areal density is ˜25 nm. The minute flux emanating from such nanoscale domains is challenging to detect with current-art sensors, on account of fundamental limitations intrinsic to sensors based on ferromagnetic materials such as magnetic noise and spin torque effects. Furthermore, current-art sector servo schemes while effective for track seeking and synchronization operations, do not permit active feedback of the positioning of the write/read head during data writing and reading. This open-loop operation is expected to be a major source of errors and reliability failures as the track width and spacing fall below 50 nm. What is needed is a sensor device for ultrahigh density magnetic recording that allows on-the-fly real time detection of written bits in order to provide precise servo information to the sensor in a closed-loop configuration.